Need some more information:
1. What exact Mac model do you have?
2. What exact Mac OS are you using? If it's a recent upgrade/installation, did you just upgrade "in place", ie, have the new OS basically overwrite what was on your Mac already?
3. This is critical: are you making backups to an external device?
4. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You can actually do a good amount of disk cleanup on your own, and there are quite a few excellent products available (both free and commercial) that can help you with those tasks.
5 What EMail client/program are you using?
6. When did this behavior start happening?
Once you provide succinct answers to these questions, we can help you.
(By the way, I use Outlook 2016 on both of my Macs running the latest version of High Sierra. I looked in the Preferences for Outlook 2016 (and any possible System Preferences), but did not see any "setting" for what happens when I click on a link in an EMail (trusted one, of course!). Maybe your machine contains some kind of malware that is causing this).